The battery problem nobody mentions in the datasheet
A standard solenoid draws current every time it moves, and if it holds position electromagnetically it draws current the entire time it’s engaged. For a grid-powered machine that’s fine. For a battery cabinet, a solar locker, or anything on a remote site, that standby draw is the difference between a service call every six weeks and one every fourteen months. Bi-stable latching solenoids exist to kill that draw. This is the short version of a conversation we have with battery-powered OEMs at least once a week.
How a standard solenoid holds
A normal solenoid is a coil around a plunger. Energize it, the plunger moves. Cut the current, a spring returns it. To hold it in the energized position you keep current flowing. A typical 24 V unit pulls 8 W while held. Across a bank of 30 compartments that’s 240 W of pure heat if they were all held at once, and even one held steadily is a steady tax on the PSU and the cooling.
How a bi-stable solenoid holds
A bi-stable (or latching) solenoid adds a permanent magnet to the assembly. The magnet holds the plunger in either end position with zero current. To move it you send a short pulse — one polarity to push out, the reverse polarity to pull back. Once it moves, the magnet holds it there. The coil is only active for the 50–100 ms of the pulse.
Standby power: zero. That single property is why we push bi-stable units for anything off-grid.
Side-by-side
| Aspect | Standard solenoid | Bi-stable latching solenoid |
|---|---|---|
| Standby power | Continuous while held (8–15 W typical) | Zero |
| Energy per cycle | Pulse + hold time × power | Two short pulses only |
| Position memory on power loss | Lost (spring returns it) | Retained (magnet holds) |
| Driver circuit | Simple (relay or MOSFET) | Needs polarity reversal; H-bridge or dual driver |
| Cost | Lower | 10–30% higher per unit |
| Best fit | Grid-powered, frequent cycling | Battery, solar, or thermal-sensitive cabinets |
The driver circuit is the catch
Bi-stable solenoids are not a drop-in replacement. You need a driver that can reverse polarity, because the same two wires carry both the open pulse and the close pulse. A simple MOSFET switch won’t do it; you need an H-bridge or a dual-driver arrangement. We supply a reference schematic with every bi-stable order, and most of our locker clients implement it on the same board that drives their LEDs. If your firmware team hasn’t done one before, budget a day, not a week.
Where we’ve actually shipped them
Our desert deployment (12-compartment solar cabinet, 20 Ah LiFePO4, 14 months on one charge cycle) used bi-stable latches on every compartment. A European bike-share dock used them on the lockout arms to survive a winter without grid power. A US medical sample locker used them so the cabinet could hold position through a building power outage and still report its state to the nurse station. The common thread: they all needed to hold a position for a long time without drawing.
When not to use one
If your machine is on mains power and cycles often, a standard solenoid is cheaper and simpler, and the standby draw is negligible against your compressor or heater load. Don’t add the driver complexity for a cabinet that’s plugged into the wall. Use bi-stable when the power budget is tight or when you need the latch to remember its position through a brownout.
Specifying a bi-stable unit
Give your supplier: voltage, stroke, holding force at the worst-case air gap, pulse duration and current, operating temperature, and the polarity sequence you plan to drive. We model the force margin the same way we do for standard latches — 30% above the measured worst-case gap. If you want the locker context, our solenoid latch spec guide covers the fail-safe vs fail-secure choice that decides whether bi-stable even fits your design.
For the broader picture on solenoids in self-service equipment, see our parcel locker and vending machine deep dive.
Written by the engineering team at Cixin Solenoid. Bi-stable latching solenoids, tubular electromagnets, and custom actuator designs for off-grid and low-power OEMs.
